First of all, Overman King Gainer. Honestly, some of the voice work was pretty good, but the main character's talentless faggot of a voice actor ruined everything. Jesus christ, it's like they hired some weeaboo 16 year old fandubber from youtube.
It's pretty obscure, but the Devilman OVA dub was the most horrible shit ever. It was like a really awful joke dub; Akira/Amon sounded like a constipated midget. Then there's the One Piece 4kids but but everyone hates that one already so there's no use mentioning it.